Thin film electroluminescence (hereinafter abbreviated as TFEL) displays having a so-called double insulation structure composed of a luminescent layer comprising a fluorescent substance containing manganese as an activator or luminescence center which is sandwiched in between a transparent electrode (indium-tin oxide, hereinafter abbreviated as ITO) and a metallic electrode via an insulating layer on each side thereof have been recognized as a promising technology for flat panel displays because of their high luminance, high resolving power and feasibility of large volume displaying.
In FIG. 7 is shown a perspective cutaway view of the main part of a conventional double insulation type TFEL display element. As shown in FIG. 7, the conventional TFEL display element is composed of glass substrate 1, ITO transparent electrode 2 (first electrode), insulating layer 3 comprising SiO.sub.2, Si.sub.3 N.sub.4, etc. (first insulating layer), luminescent layer 4, second insulating layer 5 made of the same material as the first insulating layer, and aluminum electrode 6 (second electrode).
The luminescent layer comprises zinc sulfide as a matrix having added thereto a small amount of manganese as a luminescence center. The luminescence center Mn has an optimal concentration ranging from 0.4 to 0.6% by weight based on zinc sulfide for obtaining a practical luminance of at least 100 cd/m.sup.2. Such a luminescent layer is produced by vacuum evaporation, sputtering, an ALE (Atomic Layer Epitaxy) method, etc. followed by annealing in high temperatures for dispersing manganese in the zinc sulfide matrix.
FIG. 8 shows a cross section of another example of a conventional double insulation type TFEL display element. this example, the element is sealed with silicon oil 8 and covered with sealing glass 7 for the purpose of preventing moisture in the atmosphere from entering the luminescent layer to thereby ensure the life of the element.
Where soda-lime glass is used as a glass substrate in these conventional TFEL display elements, sodium in soda-lime glass is diffused into the ITO transparent electrode, which results in an increase of electric resistance of the ITO transparent electrode, though depending on the process for preparing the ITO transparent electrode, to reduce the characteristics of the element.
In order to prevent sodium diffusion in simple matrix drive liquid crystal displays, a method of providing a barrier layer comprising SiO.sub.2, etc. between a glass substrate and a transparent electrode has been employed. This method is effective in liquid crystal displays of simple matrix drive and the like but is ineffective in TFEL displays because of involvement of a high temperature treatment.